Tokyo > Fuji > Kyoto > Osaka (Oneway journey)

Hi,

My friend and I are planning to spend 4 days in Tokyo, 2 days in Fuji (to climb Mount Fuji), 2 days in Kyoto and 2 days in Osaka in that order, in August. The days spent in each place after leaving Tokyo may change but it will stay at 6 days in total.
Our flight lands at Tokyo Airport but we leave at Osaka airport therefore a return journey to Tokyo is not needed.

Would it be worth it to get the 7 day JR Pass if we activate it on the day we leave Tokyo, even though we are not planning a return journey?

And if the pass is not needed, would be just have to buy the one-way tickets there or would we have to reserve seating somewhere? Or perhaps is there another method of travel (or train company) which is the better option?

Thanks in advance,
Vin